Physicists warm up to ultracold experiments

An article by UChicago scholars described an entirely new technique for attaining Bose-Einstein condensation.

By Steve Koppes

At 1 p.m. Friday, Nov. 9, 2007, two graduate students working in Cheng Chin’s laboratory cooled a million cesium atoms to within 10 degrees nano-Kelvin of absolute zero.

There, in the sub-basement of the Gordon Center for Integrative Science, physicists had successfully attained Bose-Einstein condensation (BEC) with cesium atoms for the first time on U.S. soil. BEC is a new state of matter formed by atoms in a gas, but only in conditions approaching absolute zero (minus-459 degrees Fahrenheit).

The phenomenon is named for Satyendranath Bose and Albert Einstein, who described the underlying theory in the 1920s. Scientists in Boulder, Colo., finally achieved BEC with rubidium atoms in 1995.

Chin, Assistant Professor in Physics, had worked toward achieving BEC in his laboratory for more than a year. Nevertheless, the result came as a surprise. “I didn’t expect it would work out so fast,” he says.

Reporting their results in the journal Physical Review A along with Chin were graduate students Chen-Lung Hung and Xibo Zhang, and Grainger Postdoctoral Fellow Nathan Gemelke. Their article described an entirely new technique for attaining Bose-Einstein condensation.

Faster Process Means Faster Results

Chin’s team has shortened the BEC process to two seconds from 15, speeding the rate of data collection. He placed a steaming cup of hot water on a table in his office to illustrate the new method.

“The evaporative cooling also occurs to hot water inside a cup. When hot water molecules leave the surface, then the remaining water is cooled down,” Chin explains. In the laboratory, the “cup” that contains the cesium atoms is a set of crossed laser beams.

In Chin’s group, making BEC is just where the fun begins. “The colder we can reach, the more exciting physics gets.” Chin says.

Since the end of 2008, Chin’s team started looking into the dynamics of ultracold atoms in a complex environment called “optical lattices,” in which atoms propagate in a way that simulates electric flows in a crystal. Here the team reported the observation on the transition from a “superconducting” to an “insulating” state.

“Understanding and controlling the fundamental process that controls conductivity is one of the major goals toward a smart, programmable material,” Chin says, referring to the development of ultra-powerful quantum computers.

Joining a Legacy

Chin established the first ultracold research program at the University when he joined the Chicago faculty in 2005. He is adding to the scientific legacy of a field that alumna Deborah Jin, PhD’95, helped pioneer.

As a graduate student at Chicago, Jin worked in Thomas Rosenbaum’s low-temperature laboratory. For her PhD project, she designed and built an experiment to study heavy fermion superconductivity.

The electrons in an ordinary metal have a velocity in excess of 1,000 miles an hour. But in fermion superconductivity, electrons move at the speed of molasses, said Rosenbaum, University Provost and the John T. Wilson Distinguished Service Professor in Physics.

Jin’s experiment carefully explored the electrons’ thermodynamic properties as they shifted into a superconducting state in which current flows without resistance. After Jin left Chicago, she introduced the same techniques she used to better understand fermionic superconductivity to the new field of ultracold gases.

Jin continued her research at JILA, an interdisciplinary institute for scientific research and graduate education in Boulder, Colo. In 1999 at JILA, She announced the transformation of a fermionic gas into yet another strange new form of matter. Science magazine listed her accomplishment as one of the year’s top 10 breakthroughs.

Jin displayed an extraordinary intellectual independence as a graduate student that has continued to serve her well, according to Rosenbaum. “If you would come up with a glib answer to what was going on, she would push back very hard, very politely, until she was satisfied. And she was often right,” he says.